The present invention relates to a calcining system. In particular, the calcining system is operated in a substoichiometric mode to reduce nitrogen oxide generation and aid in removing sulfur from the materials to be processed.
Rotary kilns are used in calcining systems to process materials, such as limestone and mineral ores. Typically, the feed material is oxidized, hardened and/or reduced by passing through the rotary kiln. The material may be preheated by a preheating device prior to entering the rotary kiln. The rotary kiln is slightly inclined on a horizontal axis and has a feed end and a discharge end. The material enters at the feed end, is processed in the rotary kiln and exits at the discharge end. The processed material exits at the discharge end where it may enter a cooling device.
During the processing of the material, combustible fuel and combustion air are introduced into the rotary kiln to oxidize the material. Current systems generate high nitrogen oxide emissions and do not remove sulfur from the materials being processed. One way of reducing nitrogen oxide emissions from a rotary kiln calcining system is to install a nitrogen oxide removal equipment at the gas exhaust end of the kiln. However, the inclusion of nitrogen oxide removal equipment is expensive and results in high operating costs. Current systems will remove sulfur from the feed material by heating the material to temperatures in excess of 2500xc2x0 F. Heating the material to such high temperatures results in lower product quality, such as a low slaking rate or low reactivity.
Combustion air, to combust a combustible fuel (or combustible material), is typically introduced into the rotary kiln at the discharge end of the kiln. Combustion air is introduced with the combustible fuel to combust the combustible fuel upstream of the air injection. When combustion air is introduced along the length of the rotary kiln it is typically introduced with combustible fuel, thereby creating upstream combustion of the combustible fuel. Upstream combustion of the combustible fuel does not aid in removal of sulfur from the feed material.
A calcining system is needed in the art for creating an oxygen deficient atmosphere in the kiln that will lower nitrogen oxide generation an aid in removing sulfur from the materials to be processed. Furthermore, the calcining system should completely combust the combustible fuel before it exits the rotary kiln.
The present invention is a calcining system operating in a substoichiometric mode for reducing nitrogen oxide generation and removing sulfur from materials to be processed by creating an oxygen deficient atmosphere. The calcining system includes a rotary kiln rotating on a slightly inclined horizontal axis. The rotary kiln has a feed end introducing material to be processed and a discharge end. The calcining system further includes a precalcining device positioned adjacent the feed end such that the material passes through the device prior to entering the rotary kiln. A stationary hood surrounds the discharge end of the rotary kiln. A burner protrudes the stationary hood. The burner introduces combustible fuel and combustion air into the rotary kiln through the discharge end and operates in a substoichiometric mode. At least one air nozzle is located along the length of the rotary kiln. The air nozzles inject combustion air into the rotary kiln. The combustion air introduced by the air nozzles completely combusts the combustible fuel introduced by the burner before the combustible fuel exits the feed end.
In another preferred embodiment of the invention, a method is used for calcining a feed material in a rotary kiln rotating on a slightly inclined horizontal axis. The method includes introducing the feed material into the rotary kiln through a feed end of the rotary kiln. Combustible fuel is introduced into the rotary kiln through a burner located in a stationary hood adjacent a discharge end of the rotary kiln such that the combustible fuel travels through the rotary kiln towards the feed end of the rotary kiln. The rotary kiln operates in a substoichiometric mode near the discharge end. Combustion air is introduced into the rotary kiln through the discharge end of the rotary kiln. Combustion air is injected into the rotary kiln between the feed and discharge ends of the rotary kiln through at least one air nozzle. The combustion air injected by the air nozzles completely combust the combustible fuel introduced by the burner before the combustible fuel exits the feed end of the rotary kiln as exhaust gas. The method creates an oxygen deficient atmosphere at the discharge end of the rotary kiln. The oxygen deficient atmosphere lowers nitrogen oxide generation and aids in removing sulfur from the feed material being processed.